The present invention relates to electrically heatable transparent panels comprising spaced electrically conductive bus strips interconnected by electrically conductive heating means deposited on a substrate of glazing material. The expression "glazing material" is used herein to denote transparent material for use in closing window openings and thus includes glass, victrocrystalline materials and plastics materials when transparent.
Such heating panels are known to be particularly useful when embodied as vehicle windows, especially rear viewing windows of motor vehicles, and for other purposes.
It will be apparent from a walk through any car park that the most common form of vehicle rear window heater comprises bus strips extending up the side margins of the window which are interconnected by a plurality of linear heating elements extending generally horizontally across the window. Depending on the height of the rear window there will often be about thirteen or fourteen of these elements spaced apart by some 2.5 to 3.5 cm and each element will be about 1 mm in width.
It is known in the case of a `wrap-around` rear window to improve the appearance of the side portions by reducing the width of the heating elements there, as compared with the width of the heating elements on the central portion, and to compensate for the resultant increase in resistance by providing conductive shunts.
These heating elements are conventionally formed by serigraphic deposition of a conductive enamel which is heat-bonded to the window. The bus strips are usually formed in the same way and indeed at the same time. It is a usual requirement that the heat output from the window heater should be 140-160 watts, and this implies a total conductivity of the heater of about 1 reciprocal ohm in a 12 volt system. Thus the average resistance of the individual heating elements should be about N ohms where N is the number of heating elements.
In practice, this resistance requirement is in conflict with a maximum permissible line-width (for visibility reasons) so that to achieve the necessary conductivity the enamel which is used is overwhelmingly, if not universally, a silver containing enamel.
Silver is very expensive.
One way of economising on the use of silver which is very often adopted is to coat the window heater electrolytically with a metal such as copper. It would clearly be advantageous to dispense with this step if possible unless it is required for some other purpose.
A further disadvantage of this most common form of heated rear window lies in the width of the heater elements. At a width of 1 mm, they are obtrusive when viewed through a rear view mirror by a driver of the vehicle.
Yet a further disadvantage lies in the relative spacing of these heater elements. Such heater elements are apt to become abraded and damaged or even broken prematurely, for example by over-enthusiastic cleaning of the window and when this happens, not only is the heat output from the heater much reduced, but the reduction is within a strip across the whole breadth of the window which often cannot be cleared of condensation for some considerable time.